In U.S. Pat. No. 3,811,317, an apparatus is disclosed for testing membrane for holes by progressively passing a roll of membrane through a chamber in which one side of the membrane is exposed to gaseous ammonia, and the other side of the membrane is exposed to a reduced pressure, to draw the ammonia through any holes present in the membrane. When ammonia does so pass through a hole in the membrane, it encounters a sensing medium such as diazo indicator paper, where the presence of the ammonia is recorded by the formation of a blue mark on the diazo paper. Accordingly, the holes in the membrane may be located, and patched if desired.
In U.S. Pat. No. 3,757,955, membrane diffusion devices such as blood oxygenators are taught in which the diffusion membrane is made of a hydrophobic material such as polypropylene or polytetrafluoroethylene. The membrane defines myriads of tiny pores, for example of approximately 1/2 micron width. This type of porous membrane has been shown to exhibit highly superior diffusion properties for carbon dioxide and oxygen, while still preventing the passage of blood across the membrane, when the diffusion device is properly constructed. Blood oxygenator devices utilizing such porous, hydrophobic membrane have recently become commercially available.
Although the diffusion membrane in such blood oxygenator devices is porous, it is still necessary that it be free of holes which are 50 to 100 microns in diameter and greater. When such holes are present, significant amounts of blood can leak from the blood side of the membrane to the oxygen side, which is of course highly undesirable. When this takes place, the entire diffusion device usually must be discarded, and a new one selected. Accordingly, prior to its assembly into an oxygenator, it is important to determine whether or not the porous, hydrophobic membrane used in an oxygenator possesses such larger holes. There is a substantial cost saving in either patching or discarding the unused, defective membrane rather than assembling the membrane into an oxygenator device, and then discovering that holes of excessive size are present.
There exists, therefore, a need for a method of detecting enlarged holes in porous membrane material which contains pores which are smaller than the holes to be detected. In the apparatus of Patent No. 3,811,317 cited above, it has been found that ammonia tends to diffuse through all holes of any size in the micronic range, which makes the detection of enlarged holes in normally porous membrane virtually impossible.
In accordance with this invention, a method and apparatus are disclosed in which porous membrane material may be scanned by a non-electrical technique for holes of larger size, even though the enlarged holes are still so small that they may not be reliably spotted by means of visual inspection.
The method and apparatus of this invention can be also used for detecting holes above a predetermined size in non-porous membrane or the like, but is the first available apparatus for detecting enlarged pin holes in normally porous membrane medium by a non-electrical technique, precisely "mapping" the exact location of each hole on a detection medium, which may be diazo indicator paper when ammonia is the detector fluid. By inspection of the detection medium, the pin hole is precisely located without further effort, and may be patched.